Reciprocating Engines

Size Range:

0.8 MW to 20 MW

Technology

Reciprocating engines operate on the same principles as petrol and diesel automotive engines. They enjoy high volume mass production and are often the lowest capital cost per kW of capacity. Reciprocating engines currently account for the majority of DE units for continuous use under 5 MWe and for back-up power. Like automotive engines, reciprocating engines can be split into two categories; compression ignition (diesel cycle) engines and spark ignition (otto cycle) engines.

Reciprocating Engine Schematic (Including Boiler)

Source: WADE, 2003

Compression Ignition Engines

Spark Ignition Engines

Compression engines are usually four-stroke direct ignition machines, often equipped with turbochargers and intercoolers. The distillate and heavy oil fuelled reciprocating engines often use the diesel cycle, which relies on the heat of compression to ignite the fuel. The piston’s compression stroke raises the pressure and temperature of the combustion air above the self-ignition temperature of oil, and then very high-pressure injectors spray a mist of atomized fuel into the hot air, causing immediate ignition, expansion and power stroke.

Spark ignition engines are a derivative of compression ignition engines, the difference being that a high intensity spark as well as compression is used to instigate the combustion. To obtain lower NOx emissions, modern spark engines use a pre-chamber to create a near stoichiometric mixture (an exact ratio with no excess of reactants) of the fuel with air. Spark ignition engines give less heat to the exhaust gas and more heat to the cooling system than compression engines.

To optimise the air-intake, modern engines use a turbocharger / compressor package that raises the pressure of the air in the intake manifold to over twice atmospheric pressure. To further increase air into the cylinder, the compressed air, which has been heated by the compression, is aftercooled. This removal of some of the heat of compression allows more molecules of oxygen to enter the pistons on the intake stroke, further increasing the fuel that can be burned on each cycle and thus almost doubling the power output compared to naturally aspirated engines. This has significantly lowered the capital cost of reciprocating engine based cogeneration systems.

Performance and Efficiency

Compression engines achieve electrical efficiencies in the range of 35-55% and size range of 75 kW-20 MW. Spark ignition engines have lower efficiencies (30% to 50%), due to the possibility of knocking - caused by over rapid combustion of fuel in the cylinder. They are also smaller in size, ranging from 15 kW to 10 MW. Up to a third of the fuel energy is available in the exhaust at temperatures from 370-540ºC, but the other rejected heat is low temperature, often too low for most processes. (Jacket cooling water at 80 to 95ºC, lube oil cooling at 70ºC and intercooler heat rejection at 60ºC, all difficult to use in CHP). Reciprocating engines operate with significantly less excess air than gas turbines, so that combustion temperatures are higher, with detrimental effects on NOx production, which does not occur below about 1300ºC. Thus, although excess air reduces engine fuel efficiency, it is usually essential to control NOx emissions. Modern engines have delayed ignition timing and increased compression ratios, which help reduce NOx without compromising power output and efficiency.

Fuel Types

Compression ignition engines generally use diesel fuel. Natural gas or evaporated gasoline is more difficult to ignite with a compression engine and depends on spark plugs to ignite the fuel and create the combustion that leads to the power stroke. Dual fuel engines can operate in the pure oil injection mode, but can burn up to 97% natural gas. Industrial applications of gas engines can use oil as fuel, though gas-engine applications would be very attractive where natural gas, LNG, or biogas is available. Spark ignition engines mostly use natural gas, though biogas and other gases can be used.

Applications

Small Industrial, Commercial, Residential

Low maximum temperatures in the cooling system limit useful heat recovery. However, in some cases where technical difficulties have been overcome, supplementary firing can be used to increase the quality of heat.

Depending on size, reciprocating engines can be used to produce up to 15 bar of steam from the exhaust gases with independent production of hot water at 85-90ºC from the cooling system;

If the heat from the exhaust gases and cooling systems are combined it is possible to produce water at 100ºC and steam at higher temperatures;

The exhaust gases can also be directly recuperated and used for drying or CO2 production. All residual energy from the engine can be used to produce hot air.

Reciprocating engine cogeneration is typically applied in buildings and institutional settings, and less frequently for industrial use.

Advantages and Disadvantages

Advantages

Disadvantages

Lowest first cost of all CHP systems;

High efficiencies at part load operation give users a flexible power source allowing for a range of different applications;

Short start-up times to full loads (10-15 seconds), make reciprocating engines suitable for backup power systems and peak shaving applications;

High reliability.

Relatively high vibrations in reciprocating engines require support and special foundations with shielding to reduce noise;

Through time reciprocating engines have achieved low initial capital costs, strong O&M support networks and high partial load efficiency. They achieve greatest economic benefits when used in small to medium size applications from 1 kW-5 MW. As the table above shows, the installed cost for reciprocating engines is between $900-1,300/kW. The figure below shows the breakdown. Reciprocating engines have a large number of moving parts, increasing all-in maintenance costs to over $10/MWh, compared to $4.5/MWh for gas turbines.